A known inhaler for inhalation into the airway of a user has a housing at least partially defining a flow passageway extending through the inhaler from an air inlet to an outlet. A pressurised metered dose canister may be placed in the housing and, when a user sucks on a mouthpiece of the housing, air may be drawn into the housing between the canister and an inner wall of the housing and may flow along past the canister towards the outlet. The canister includes a valve stem and a metering valve arranged to seat in a valve stem block formed on the housing and a main canister body of the canister may be moved relative to the housing and valve stem so as to operate the metering valve and fire a metered dose of propellant and active drug through the valve stem block and into the flow passageway.
This type of inhaler often works well, but some users may find it difficult to coordinate an inward breath for inhalation with firing of the canister and, if the canister is fired too early or late relative to an inhalation breath, optimum inhalation may not be achieved.
In WO 2009/128491, which is incorporated by reference herein, a vinyl bag with holes in it is attached to an inhaler body to provide air flow resistance.
In WO 98/41252, which is incorporated by reference herein, an inhaler includes an end cap and housing and the end cap is slideable relative to the housing between a configuration in which there are respective minimum and maximum air flows through the housing. The housing has to be specially formed with a rim and a special collar needs to be located between the housing and end cap and a movement of the collar relative to a dispensing container of the device is undesirably required during an initial setting up of the device. It may also be possible for fluff or debris to become lodged but difficult to sense near the air inlets of the device near the collar, potentially allowing inhalation thereof, and the air inlet area is limited by narrow gaps between the collar and housing.
WO 2008/040062, which is incorporated by reference herein, discloses an inhaler with a cover cap arranged to seal the inhaler so that nothing may enter the inhaler through a housing top in order to exclude mud and dust and for use in wet conditions. The cover cap is flexible to allow operation of the device's canister without allowing mud or dust on the cap to be sucked in and inhaled. However, this requires a complicated arrangement of moving parts elsewhere on the housing in order to provide an air inlet. It is known to provide a breath-actuated inhaler in which the act of inhalation by a user causes a dose to be provided, but this type of device is fairly complicated and expensive.